| Message ID | 20230302195618.156940-18-john.ogness@linutronix.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | threaded/atomic console support | expand |
On Thu 2023-03-02 21:02:17, John Ogness wrote: > Invoke the atomic write enforcement functions for rcu stalls to > ensure that the information gets out to the consoles. "ensure" is too strong. It is still just the best effort. It might fail when the current console user does not pass the lock. I would say that it will increase the chance to see the messages on NOBKL consoles by printing the messages directly instead of waiting for the printk thread. > It is important to note that if there are any legacy consoles > registered, they will be attempting to directly print from the > printk-caller context, which may jeopardize the reliability of the > atomic consoles. Optimally there should be no legacy consoles > registered. The above paragraph is a bit vague. It is not clear how exactly the legacy consoles affect the reliability, Does it mean that they might cause a deadlock because they are not atomic? But there is nothing specific about rcu stalls and priority of NOBKL consoles. This is a generic problem with legacy consoles. > --- a/kernel/rcu/tree_stall.h > +++ b/kernel/rcu/tree_stall.h > @@ -566,6 +568,8 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps) > if (rcu_stall_is_suppressed()) > return; > > + prev_prio = cons_atomic_enter(CONS_PRIO_EMERGENCY); Thinking loudly: This would set the EMERGENCY priority on this CPU. But the following function: + rcu_dump_cpu_stacks() + dump_cpu_task() + trigger_single_cpu_backtrace() might send IPI and the backtrace will be printed from another CPU. As a result that backtraces won't be printed with EMERGENCY priority. One solution would be to have also global EMERGENCY priority. Another possibility would be to use EMERGENCY priority also in nmi_cpu_backtrace() which is the callback called by the IPI. I would probably go for the global flag. printk() called in EMERGENCY priority has to flush also messages added by other CPUs. So that messages added by other CPUs are printed "directly" anyway. Also setting the EMERGENCY priority in nmi_cpu_backtrace() is an ad-hoc solution. The backtrace is usually called as part of another global emergency report. > + > /* > * OK, time to rat on our buddy... > * See Documentation/RCU/stallwarn.rst for info on how to debug Best Regards, Petr
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h index 5653560573e2..25207a213e7a 100644 --- a/kernel/rcu/tree_stall.h +++ b/kernel/rcu/tree_stall.h @@ -8,6 +8,7 @@ */ #include <linux/kvm_para.h> +#include <linux/console.h> ////////////////////////////////////////////////////////////////////////////// // @@ -551,6 +552,7 @@ static void rcu_check_gp_kthread_expired_fqs_timer(void) static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps) { + enum cons_prio prev_prio; int cpu; unsigned long flags; unsigned long gpa; @@ -566,6 +568,8 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps) if (rcu_stall_is_suppressed()) return; + prev_prio = cons_atomic_enter(CONS_PRIO_EMERGENCY); + /* * OK, time to rat on our buddy... * See Documentation/RCU/stallwarn.rst for info on how to debug @@ -620,6 +624,8 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps) panic_on_rcu_stall(); rcu_force_quiescent_state(); /* Kick them all. */ + + cons_atomic_exit(CONS_PRIO_EMERGENCY, prev_prio); } static void print_cpu_stall(unsigned long gps)
Invoke the atomic write enforcement functions for rcu stalls to ensure that the information gets out to the consoles. It is important to note that if there are any legacy consoles registered, they will be attempting to directly print from the printk-caller context, which may jeopardize the reliability of the atomic consoles. Optimally there should be no legacy consoles registered. Signed-off-by: John Ogness <john.ogness@linutronix.de> --- kernel/rcu/tree_stall.h | 6 ++++++ 1 file changed, 6 insertions(+)